The present invention relates to initiating devices for combustible materials and, more particularly, to initiator headers used to initiate the detonation of propellants, pyrotechnics, explosive materials and the like.
A wide variety of devices are used to detonate or ignite explosives and pyrotechnic materials.
One type of device commonly referred to as an initiator header, generally includes one or more conductive pins, surrounded by an insulating layer. The one or more conductive pins generally terminate at a thin bridge wire disposed along a top surface of the insulating layer. When an electric current is passed through one of the conductive pins, the current passes through the bridge wire which rapidly heats due to its electrical resistance. The heat generated by the bridge wire ignites a propellant which, in turn, ignites a gas-generating composition. Combustion of the gas-generating composition results in the production of a gas which may be used to perform a task such as inflating an automobile air bag or ejecting a pilot from a combat aircraft.
For many years, automobile manufacturers have used initiator headers as part of automobile air bag safety systems, such as those described in U.S. Pat. Nos. 3,723,205 and 4,981,534. As is well known to those skilled in the art, air bag systems are designed to rapidly inflate sealed bags with an inflatable gas when a sensor detects an automobile collision. Due to the nature of their use, air bag systems may never be used or may remain idle for many years. Despite these extended periods of inactivity, air bag systems must operate properly when they are needed. Of equal importance is the fact that air bag systems must not inadvertently deploy, a highly undesirable event that could result in death or severe injury.
There have been many efforts directed to producing low cost and easily assembled initiator headers. For example, U.S. Pat. Nos. 5,230,287 and 5,431,101 to Arrell, Jr. et al. disclose an electronically-activated initiator formed by welding a hollow cap containing an explosive material to an initiator header. The outer diameter of the initiator header slightly exceeds the inner diameter of the hollow cap so that, as the two are joined together, the header and cap form a reliable, weldable joint.
U.S. Pat. No. 5,243,492 to Marquit et al. discloses a process for making an initiator device having a centrally located conductive pin.. The pin can be electrically connected to an outer portion of the device via a bridge wire, thereby forming a device that is particularly useful for igniting a gas-generating composition in an air bag safety system.
U.S. Pat. No. 5,793,476 to Bailey discloses an electronically activated initiator header for an air bag inflator. The initiator header is ignitable by thermal energy from a resistive element formed on a semiconductor substrate. The semiconductor substrate is secured to the initiator header by a glass-to-metal seal.
In spite of the above-mentioned improvements in initiator devices, there remains a need for a reliable initiator header which is inexpensive, has fewer parts, and is easier to assemble.
In accordance with certain preferred embodiments of the present invention, an initiator header subassembly includes an electrically conductive grounding tab having an outer perimeter. The grounding tab is preferably made of a conductive material, such as metal. In highly preferred embodiments, the grounding tab is made of an alloyed metal, such as the alloyed metal commonly sold under the trademark KOVAR(copyright). The grounding tab may comprise a substantially flat plate having a top surface, a bottom surface and two holes extending between the top and bottom surfaces. In certain preferred embodiments, the grounding tabs are produced by first punching or stamping a plurality of coin-like grounding tabs from a sheet of conductive material. The conductive material may include a metal such as KOVAR(copyright) alloyed metal. Each grounding tab is then further processed to form the two holes extending between the top and bottom surfaces thereof. In certain preferred embodiments, a first one of the two grounding tab holes has a first diameter and a second one of the two grounding tab holes has a second diameter that is greater than the diameter of the first grounding tab. The entire process of producing the grounding tabs and then producing the holes extending therethrough may be done using a machine tool which punches or stamps the structure.
The initiator header subassembly of the present invention also desirably includes a mass of an insulating material that is secured atop the top surface of the grounding tab. The mass of an insulating material preferably has a top surface, a bottom surface and a sidewall extending between the top and bottom surfaces. After the mass of an insulating material has been secured to the grounding tab, the sidewall of the insulating mass is preferably in substantial alignment with the outer perimeter of the grounding tab. In other embodiments, the sidewall of the insulating mass defines the outer perimeter of the initiator head subassembly.
The mass of an insulating material may be an insulating disc preform having two longitudinal openings extending from the top surface to the bottom surface of the insulating disc. The two longitudinal openings desirably have a substantially similar diameter. As used herein, the term xe2x80x9cinsulating disc preformxe2x80x9d means that a plurality of insulating discs may be mass-produced and stored for later use during final assembly of initiator headers. As such, each of the insulating disc preforms may be substantially similar in shape, composition and appearance so that the insulating discs are interchangeable with one another. In certain preferred embodiments, the insulating disc is an all-glass insulating disc having a relatively high hardness rating capable of withstanding up to 20,000 psi or more. The insulating disc may be made of such glasses as borosilicate glass.
The initiator header subassembly may also include a pair of electrically conductive pins extending completely through the grounding tab and the insulating disc. The conductive pins preferably pass through the respective first and second holes of the grounding tab. In certain embodiments, a first one of the electrically conductive pins is attached to and electrically interconnected with the grounding tab and a second one of the electrically conductive pins is electrically isolated from grounding tab. The electrically conductive pins may be made by cutting sections of wire, having a predetermined length from a spool of metal wire, such as KOVAR(copyright) alloyed metal. After a plurality of such predetermined lengths have been cut from the spool, the pins are placed in a tumbler which rounds off the ends of the pins. The pins may also be machine tooled to round off the ends.
A fixture may be used for assembling one or more of the above-described initiator header subassemblies. On the other hand, the outer diameter of the electrically conductive pins closely matches the inner diameter of the two longitudinal openings extending through the insulating disc. The fixture preferably holds all of the components of the initiator assembly in proper orientation relative to one another during the assembly process. This may be accomplished by matching the diameter of the outer surface of the first pin with the diameter of the first hole through the grounding tab, while the second grounding tab hole has a greater diameter than the first hole. In certain preferred embodiments, the fixture includes a lower member having one or more cavities for receiving the various components of the initiator header subassembly and an upper member or cap that is securable atop the lower fixture member. In one particularly preferred assembly method, the electrically conductive pins are cut to predetermined lengths as described above. The pins are then exposed to a chemical etchant that roughens or pits the outer surface of the pins. Although the present invention is not limited by any particular theory of operation, it is believed that pitting the outer surface of the pins will increase the exposed surface area of the pins, which will enhance the strength of the glass-to-metal seal between the conductive pins and the insulating disc. The pins are then subjected to an oxidizing process which forms a thin oxide layer atop the pitted outer surface of the pins. The pins are then positioned within the cavity of the lower fixture member. The cavity preferably holds the pins in a substantially parallel orientation relative to one another. In one preferred embodiment, the cavity of the fixture holds the pins in a substantially vertical orientation relative to a table top supporting the fixture.
A grounding tab is then positioned over the upper ends of the pins and moved in a downward direction so that the pins pass through the holes in the grounding tab. The cavity of the fixture preferably includes a shelf which holds the grounding tab at an elevation located between the upper and the lower ends of the two conductive pins. As mentioned above, the diameter of the first grounding tab hole substantially matches the diameter of the outer surface of the pin so that the pin and first grounding tab hole are in close contact with one another. The diameter of the second grounding tab hole is preferably larger than the diameter of the outer surface of the second conductive pin passing therethrough so that the second grounding tab remains electrically isolated from the second conductive pin. A metallic washer or ring may be placed around the first conductive pin and held by the fixture adjacent an upper or lower surface of the grounding tab. The metallic washer or ring may include a copper-silver alloy which creates an intermetallic bond or braze joint between the first conductive pin and the first hole extending through the grounding tab. Before the grounding tab is placed in the cavity of the lower member of the fixture, the grounding tab may be exposed to a chemical etchant which roughens or pits the outer surface of the grounding tab. The grounding tab is then subjected to an oxidizing process which forms an oxide layer over the pitted outer surface of the grounding tab.
An insulating disc may then be placed in the cavity of the fixture and atop the grounding tab so that the longitudinal openings of the insulating disc are in substantial alignment with the first and second holes of the grounding tab and so that the conductive pins pass through the two longitudinal openings. The upper member of the fixture is then placed atop the lower member of the fixture and the fixture is placed in a furnace which heats the assembly to approximately 1700-1900xc2x0 F. When the components of the initiator header subassembly are heated, the all-glass insulating disc transforms from a solid state into a molten state so as to facilitate the formation of a glass-to-metal seal with the outer surface of the conductive pins and the top surface of the grounding tab. The fixture may then be removed from the furnace and placed on a cooling rack.
In certain preferred embodiments, the grounding tab and the electrically conductive pins have a substantially similar coefficient of thermal expansion. In yet other preferred embodiments, the grounding tab, the insulating disc and the electrically conductive pins all have a substantially similar coefficient of thermal expansion.
After assembly of the initiator header subassembly, the upper ends of the two electrically conductive pins are preferably accessible at the top surface of the insulating disc. An electrically conductive element, such as a bridge wire or a semiconductor bridge, may then be attached to the upper ends of the two electrically conductive pins exposed at the top surface of the insulating disc. In other embodiments, the bridge wire or semiconductor bridge may not be placed directly on the top surface of the insulating disc, but may be spaced from the top surface so as to span a gap between the two electrically conductive pins.
In the final assembly, the first electrically conductive pin is attached to and electrically interconnected with the first grounding tab hole. The second conductive pin is preferably spaced from the inner diameter of the second grounding tab hole so that the second conductive pin is electrically isolated from the grounding tab. As such, the pair of electrically conductive pins pass through the aligned longitudinal openings of the insulating disc and the two holes extending through the grounding tab.
In certain preferred embodiments, the grounding tab may include a sealing flange that preferably extends or projects from the bottom surface of the grounding tab. The sealing flange extends in a direction that is substantially perpendicular to the top surface of the grounding tab. The sealing flange preferably has an outer perimeter that substantially matches the outer perimeter of the grounding tab. A hollow cap filled with an explosive material may be secured atop the initiator header subassembly. The hollow cap preferably has a closed end and an open end. To assemble the initiator header with the cap, the upper ends of the conductive pins are first inserted into the open end of the hollow cap. The inner diameter of the hollow cap preferably has a diameter that closely matches the outer diameter of the initiator header, the outer perimeter of the grounding tab and/or the outer perimeter of the sealing flange.
In other preferred embodiments, an initiator header subassembly includes an electrically conductive grounding tab having a top surface, a bottom surface and two holes extending between the top and bottom surfaces, the grounding tab having an outer perimeter. The subassembly also includes an insulating disc secured atop the first surface of the grounding tab. The insulating disc desirably has a top surface, a bottom surface, an outer peripheral surface extending between the top and bottom surfaces of the insulating disc, and two longitudinal openings extending between the top and bottom surfaces, the longitudinal openings being in substantial alignment with the two holes extending through the grounding tab. Two electrically conductive pins preferably extend completely through the holes in the grounding tab and the longitudinal openings in the insulating disc. A first one of the electrically conductive pins is electrically interconnected with the grounding tab and a second one of the electrically conductive pins is electrically isolated from the grounding tab. The peripheral surface of the insulating disc is in substantial alignment with the outer perimeter of the grounding tab. In other embodiments, the peripheral surface of the insulating disc defines the outer perimeter of the initiator header subassembly. In the above-described assembly, the grounding tab, the insulating disc and the electrically conductive pins preferably have a substantially similar coefficient of thermal expansion. The grounding tab, electrically insulating disc and electrically conductive pins are desirable secured together using one or more glass-to-metal seals. The insulating disc preferably is an all-glass disc, such as borosilicate glass.
In still another preferred embodiment of the present invention, an initiator header subassembly includes a conductive grounding tab having an outer perimeter, and an insulating disc secured atop the conductive grounding tab. The insulating disc desirably has an outer peripheral surface that is in substantial alignment with the outer perimeter of the grounding tab. The initiator header subassembly also preferably includes a pair of electrically conductive pins extending completely through the conductive grounding tab and the insulating disc. A first one of the pins is electrically interconnected with the grounding tab and a second one of the pins is electrically isolated from the grounding tab. The pair of electrically conductive pins are preferably substantially parallel with one another. The grounding tab may be made of a conductive material, such as KOVAR(copyright) alloyed metal. The insulating disc may be made of glass capable of withstanding up to 20,000 psi or more. In certain preferred embodiments, the insulating disc is made of borosilicate glass.
These and other preferred embodiments of the present invention will be described below in more detail.